The number of anti-L antibody binding sites on LK goat red blood cells will be determined. Anti-L antibody, raised in sheep with high potassium (HK) red cellsby injecting low potassium (LK) red cells, is capable of stmulating the Na-K pump rate in LK goat red cells. The stimulation is accomplished by altering the affinity of the LK cell for intracellular potassium; intracellular potassium inhibits the activity of the Na-K pump. In order to determine whether the antibody exerts its effect by combining directly with the pump or by combining with some other membrane component and so altering the environment of the pump, the number of antibody molecles bound per cell will be counted using I131 labelled antibody, and the number bound will be compared with the number of pump sites as determined by the number of ouabain molecules bound. In human red blood cells, theeffect of intracellular Na on the K sub 1/2 of the pump of human red blood cells will be determined as will the effect of extracellular K on the K sub 1/2 of the pump for Na. Such measurements will permit evaluation of the simultaneous model for the Na-K pump.